First night, for example, initially lost one guy; so careless (9)
I believe the answer is:
negligent
'careless' is the definition.
(associated in meaning)
'first night for example initially lost one guy' is the wordplay.
'first night' becomes 'n' (lst letter of 'night').
'for example' becomes 'e.g.' (short for 'exempli gratia', 'for example' in Latin).
'initially lost' becomes 'li' (I can't justify this - if you can you should believe this answer much more).
'one guy' becomes 'gent' (I've seen this in another clue).
'n'+'eg'+'li'+'gent'='NEGLIGENT'
'so' is the link.
Can you help me to learn more?
(Other definitions for negligent that I've seen before include "sloppy" , "Not minding" , "Failing to take proper care" , "Not taking due care" , "slack" .)